OKLAHOMA CITY (AP) -- In an unusually early and strong warning, national weather forecasters cautioned Friday that conditions are ripe for violent tornadoes to rip through the nation from Texas to Minnesota this weekend.

As states across the middle of the country prepared for the worst, storms were already kicking off in Norman, Okla., where a twister whizzed by the nation's tornado forecasting headquarters but caused little damage.

It was only the second time in U.S. history that the Storm Prediction Center issued a high-risk warning more than 24 hours in advance, said Russ Schneider, director of the center, which is part of the National Weather Service. The first time was in April 2006, when nearly 100 tornadoes tore across the southeastern U.S., killing a dozen people and damaging more than 1,000 homes in Tennessee.

This weekend's outbreak could be a "high-end, life threatening event," the center said.

The strongly worded message came after the National Weather Service announced last month that it would start using terms like "mass devastation," "unsurvivable" and "catastrophic" in warnings in an effort to get more people to take heed. It said it would test the new warnings in Kansas and Missouri before deciding whether to expand them to other parts of the country.

Friday's warning, despite the dire language, was not part of that effort but just the most accurate way to describe what was expected, a weather service spokeswoman said.

It's possible to issue earlier warnings because improvements in storm modeling and technology are letting forecasters predict storms earlier and with greater confidence, said Chris Vaccaro, a spokesman for the National Weather Service. In the past, people often have had only minutes of warning when a siren went off.

"We're quite sure tomorrow will be a very busy and dangerous day in terms of large tornadoes in parts of the central and southern plains," Vaccaro said. "The ingredients are coming together."

The worst weather is expected to develop late Saturday afternoon between Oklahoma City and Salina, Kan., but other areas also could see severe storms with baseball-sized hail and winds of up to 70 mph, forecasters said. The warning issued Friday covers parts of Illinois, Iowa, Kansas, Missouri, Nebraska, Oklahoma and Texas.

The weather service confirmed a tornado touched down about 4 p.m. Friday near the University of Oklahoma campus in Norman, where it is based. Non-essential personnel at the storm center and students were ordered to take shelter, officials said.

Video from television helicopters showed several buildings damaged in the city of about 100,000 about 20 miles south of Oklahoma City, but Oklahoma Department of Emergency Management spokeswoman Keli Cain said there were no reports of serious injuries.

"This is just a fraction of what's to come tomorrow," Vaccaro warned.

Storms were developing as cold air from the west hit low-level moisture coming up from the Gulf of Mexico. The difference in wind direction and speed was creating instability in the atmosphere that can spawn tornadoes, said Scott Curl, another weather service meteorologist.

Emergency management officials in Kansas and Oklahoma warned residents to stay updated on weather developments and create a plan for where they and their families would go if a tornado developed.

"We know it's a Saturday and that people are going to be out and about, so stay weather aware," Cain said. "Have your cell phone on you, keep it charged and make sure you're checking the weather throughout the day so you don't get caught off guard."

People also should put together an emergency preparedness kit that includes a pair of boots, rain gear, flashlight, battery-operated radio, first-aid kit and a few days' supply of food and water.

"It seems like it's kind of a big deal this time," said Monte Evans, a 42-year-old middle school teacher in Wichita, Kan., who said he planned to keep a close eye on the weather and take shelter in his basement with his wife and four children, ages six to 11, if tornadoes hit. "But they always say it's coming and then ends up somewhere else. You just do the best you can and get ready if it happens."

Medical officials in Oklahoma warned residents not to seek shelter at hospitals or other public buildings, but rather to stay inside their homes in a basement or interior closet.

During a tornado outbreak last spring, hundreds of residents packed Oklahoma City hospitals seeking shelter from a violent series of twisters that killed seven people in Oklahoma and Kansas.

"We had people actually lining the halls," said Michael Murphy of the Emergency Medical Services Authority. "Had we experienced a mass casualty incident, it really could have placed a strain on our resources."

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